More than 1,800 flights within, to or out of the U.S. were canceled Tuesday as extreme winter weather continued to wreak havoc for travelers.

Data from the tracking group FlightAware also showed more than 4,700 U.S. flights were delayed as of Tuesday afternoon.

Southwest Airlines had the most cancellations at 391, FlightAware data showed. United was next with 338. SkyWest, Indiana-based Republic and American Airlines also each had more than 100 flights canceled.

Travelers were stranded at airports across the U.S., with New York-LaGuardia, Washington-Reagan International and O’Hare International airports leading the way.

The weather-related cancellations come on top of pre-announced ones by Alaska Airlines, which said Friday that dozens of flights would remain canceled through Tuesday as a result of the ongoing investigation into Boeing’s 737 Max 9 aircraft.

As a result, Alaska had the highest rate of cancellations Tuesday with 16% or 96 total, FlightAware data showed.

Separate data from the flight-tracking site Anuvu showed just 36% of all U.S. carriers were departing on time Tuesday.